On a good day, Carrie Fisher enjoys herself the way only a manic depressive can. "Gaaaad," she exclaims, flopping into a chair in the suite of a Dublin hotel. "I just got my hair done but ONLY because I can't do it myself, not out of vanity." Today is a good day. Yesterday was not such a good day. If we had done the interview yesterday, says Declan, her publicist, things might have been - he averts his eyes - difficult. "So I was watching the TV in my hotel room," roars Fisher, "and there was this show on and swear to God it was in the language of the elves." She pitches in her chair as if she is stabilising the roll of a ship. "What the hell language is that? I know it's not Welsh, I know it's not Latvian."

"It's called Gaelic," says Declan stonily.

"Really? People actually speak that?"

Fisher in this mood is like a throwback to that bygone age when stars occasionally slipped the PR net and drank whisky and railed to eager journalists about their lousy ex-husbands. Today she is on stupendous form, a 47-year-old in a defiantly short skirt - her stance against the evil agents of anorexia - and a gob on her like the Wife of Bath's, which seems at every blast to affirm: push me in the fire and I'll walk straight through it. Over the course of an hour she does indeed take on her lousy ex-husband, her sister-in-law's lousy ex-husband, her crazy mother, her "schmuck of a father" who, she reveals, is sleeping with a yoga teacher 20 years his junior, and all the "creepy" people who populate Hollywood. "Shit, now I've said too much," she drawls at the end, although she hasn't seriously maligned anyone. "You're gonna write a piece making fun of me, right? Taking the piss out of manic depression?" On the contrary, I say, the Guardian is the manic depressive's friend, in fact there are probably quite a few on the staff. "You'd better believe it," says Fisher, and before she can stop herself she has reeled off three big-name, closet depressives, a director and two actors who are variously treating their problem with "dope, dope and lithium". Fisher shoots me an incendiary look. "Now THAT is off the record."

Her third book The Best Awful, surely one of the worst titles in publishing history, rolls off the page in exactly the same tone of voice; super-bitchy, witty and less of a novel than an extended piece of therapy, since it is based on the unhappy story of Fisher's husband, the agent Bryan Lourd, leaving her for a man. "It would be a pretty profound failing to have someone leave you for a man, right?" she croaks. "I actually had an interviewer ask me, wouldn't it be better to be left for a man, because it's, like, NOT PERSONAL? You know, like in the Godfather? 'Tell Michael it was business.' Actually, no." She's yelling again. "I DIDN'T LOOK AT IT THAT WAY. That wasn't funny to me for a long time."

It is, however, funny to her now, partly because the couple's 11-year-old daughter Billie needs it to be so, and partly because, after years of manic depression, Fisher has come to the conclusion that it is better to laugh than to cry. This is a brave route littered with the bones of people who couldn't quite pull it off. Fisher's friend Roseanne was for years a successful proponent of the smile-though-your-heart-is-breaking school, and then there is Liza Minnelli, to whom the cost of keeping cheerful has evidently been too dear. Fisher, on today's evidence, is more robust than that, although I wouldn't like to see her in a black mood.

"The one's who don't laugh," she says, "- and I know plenty - are heavy and devastated and they're having a shit time. Whereas I'm like, get me to the funny bar. It's Chicken Soup for the Fag Widow! If I can be funny, thank you, please God, then let me. I don't want to be a victim. And I have a child, in which case it's best that everything's out in the open. It wasn't always like that. I grew up around a little ... unpleasantness."

When she was a year old, her father, the entertainer Eddie Fisher, left her actress mother Debbie Reynolds for Elizabeth Taylor. Fisher never visited; Reynolds took to drink. "That was my adolescence." Years later, Eddie wrote a memoir which he described as "an extraordinary life, worth telling", and his daughter describes as "a classy tale of all the women he'd ever screwed and how they were in bed. You know, everything you want your dad to write about."

By the time she was 30, Carrie Fisher had tried drink, drugs, an overdose and a spell in mental hospital, the common pit-stops of children of the famous. Not that she blames her parents; in fact, she says, she despises people who whine about their parents into adulthood. "In a weird way, I don't think I do badly with what I'm given. I'm not running around saying, 'boo hoo'. Like, the other day I had my mother say to me about someone, 'Well, you know, the town turned its back on her.' And I said, that's the big tragedy? If you go into showbusiness, it's going to happen sooner or later. It's going to happen TO ALL OF THEM."

Fisher thinks Hollywood is "creepy" and celebrity is "disgusting", a position she has held since childhood. She never had any intention of becoming famous. But then, she says, she "fluked" her way into the role of Princess Leia in Star Wars, one thing led to another and before she knew it, she'd fallen for its charms and was "serving time as a celebrity".

"Two of the saddest words in the English language are, 'What party?' And LA is the 'What party?' capital of the world. Everyone is sucking up. My mother's career was over at 40 but she was still trying to be everyone's buddy, always smiling for the cameras." She pulls a terrifying grin. "I just found it a little frightening."

Fisher's mother now lives next door to her in LA. If it was up to her, says Fisher, she would live as far away from the place as possible - in London, say - but Billie's father lives in LA and having been without a dad herself, she won't deny her daughter one. So, I ask hesitantly, is Eddie Fisher still alive?

"Oh yeah," she rolls her eyes. "He really is. But I hardly ever know it. He's a very strange guy, my father. I can't get mad at him because he's so adorable." She says this with heavy sarcasm and I ask what she means. "When you are in a room with him, you are the greatest thing he's ever met; you're so amazingly HOT and FUNNY and SMART. And then he could walk into the hallway and see a jacket he likes and he wouldn't remember even having run into you."

Does she let him see his granddaughter? "Actually, he rang the other day and said he was coming over to see Billie. And I waited and waited and waited. It was my whole childhood relived. By the end of the day I called my brother and I said, 'Well, you're never gonna guess who didn't come see me today!' And it's all the more heartbreaking, because YOU LOVE HIM. I'll be 75 years old and still saying, 'Daddy's coming! Make up the bed! Kill the hog!' How stoopid is that?" She summons all her energies. "Schmuck!"she spits, and falls about laughing.

Fisher has high hopes for her daughter Billie, who shows every sign of not wanting to go into showbusiness. Billie associates fame with her parents and consequently finds it mortifying; she hates it when people ask if she's Princess Leia's daughter. The only part of her mother's background she digs is the time she spent in the mental hospital. This, says Fisher wryly, has been designated by Billie's friends as "way cool". Her one regret is that Billie's eminently sensible attitude doesn't soften a little when it comes to her grandmother.

"She doesn't get it about Debbie - you know, Singing in the Rain and all that. And she probably won't until it's too late. And that's a shame because my mother is hilarious and great and Billie just thinks of her as..."

Weird grandma?

"Very weird. It's sad. Listen to what happened - I'm not even sure I should tell you this, it's so sad: there was a grandparents day at school and Billie didn't tell us. I said, 'Oh baby'. That hurt me so much for my mom. But my mother is not a regular grandmother; she's going to show up in a wig. And then my father did show up, at Bryan's behest, and my daughter FREAKED. He was wearing orange clothes and bright blue shoes and dyed black hair and he started singing when he saw her. She's 11 and super hip. So she runs up to Bryan and hisses, 'He's making a fool out of me!' with real tears in her eyes. And I call my brother later and say, 'You know how we would've hacked off an arm to get our father to show up at school? He goes to Billie's thing, and she's mortified.'" She shakes her head. "Perfect."

I ask whether Fisher still holds a grudge against Liz Taylor. She cackles. "I was at a party once and someone came up and said, 'Elizabeth is very hurt that you haven't come and said hello to her.' I mean, CHRIST, only in Hollywood. She steals my father and I have to make the first move? So I went up and she said to me -" Fisher re-enacts the conversation:

Taylor: I heard your book is really good. [that's her first book, Postcards from the Edge].

Fisher: Well, it's about alcoholism and you were in Betty Ford, weren't you, and so was my father.

Taylor: How is your father?

Fisher: I wouldn't know, I didn't really see him growing up.

Taylor: Maybe you didn't miss that much.

Jeez, I say. "I know," she says, "isn't it great? And then I saw her another time and told her I'd heard she'd been bad-mouthing my mother. And people don't talk to Liz Taylor like that. So she says to me, 'I'm going to push you in my pool.' I said, OK. She said, 'You'll pull me in with you.' I said, 'No I won't'. I said, 'PUSH ME IN THE POOL'. And she did." It turned out to be cathartic for both of them. "In some stupid way, I was telling her that she couldn't hurt me any more. We have been great friends ever since."

Debbie Reynolds doesn't date any more. "My mother says 'the store is closed' and I say to her, I just wanna know, what was for sale?" Eddie Fisher is seeing his yoga teacher. And Carrie Fisher is single. Who, I wonder, has been the love of her life?

"Ugh," she says and falls silent for a record two seconds. "It's gonna have to be, in some freaked out planet of darkness, Paul Simon. [They were married for 13 years.] And a little bit of Bryan; the people I spent time with. But there really hasn't been one. Not really."

The Best Awful, it turns out, refers to the experience of mania, "which is enormous fun until it becomes way too much fun. Then it's too much of a good thing." Fisher thinks Hollywood is just as cruel and hypocritical as it always was. But there is something to be said for the gentler language of her parents' age, the pre-clinical speak that encouraged a stoicism you don't often see in what she calls whingeing celebrities. "The old terms sounded like things that could just kinda happen to you in a bar. He's 'melancholic'. You know? But these new ones they come up with - dysphoria, bipolar, manic depression - I mean," she looks her devilish best, "who'd want to be any of those?"

· Carrie Fisher will be appearing at The British Library at 6.30pm tonight as part of the Orange Word Season; for tickets call 0207-412 7332. The Best Awful is published by Simon and Schuster at £14.99.